Adding package upload to the Packages API docs

What does this MR do?

Docs don't cover how to upload packages. It might be good to expand this a bit further with a "This is for generic packages - please refer to Package Registry for Maven/NPM/etc."

Today I was looking for documentation on how a customer would upload packages since I know I've seen it before; it's odd that the upload functionality isn't covered in our API docs.

Related issues

N/A

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